Notes

Conditions thermally and chemically similar to those on Earth have presumably existed on other bodies in the solar system. Venus, Mars, Europa (a moon of Jupiter) and Titan (a moon of Saturn) have for example all probably had liquid water at some time. But there is so far no evidence for life now or in the past on any of these. Yet if life had arisen one might expect it to have become widespread, since at least on Earth it has managed to spread to many extremes of temperature, pressure and chemical composition. On several occasions structures have been found in extraterrestrial rocks that look somewhat like small versions of microorganism fossils (most notably in 1996 in a meteorite from Mars discovered in Antarctica). But almost certainly these structures have nothing to do with life, and are instead formed by ordinary precipitation of minerals. And although even up to the 1970s it was thought that life might well be found on Mars, it now seems likely that there is nothing quite like terrestrial life anywhere else in our solar system. (Even if life is found elsewhere it might still have originally come from Earth, say via meteorites, since dormant forms such as spores can apparently survive for long periods in space.)

Away from our solar system there is increasing evidence that most stars have planets with a distribution of sizes—so presumably conditions similar to Earth are fairly common. But thus far it has not been possible to see—say in planetary atmospheres—whether there are for example molecules similar to ones characteristically found in life on Earth.